Mineral ‘fingerprints’ to aid more cost-effective exploration

By
Patrick Nadoll*
18 June 2014

Editorial

Exploration
geologists face the challenging task of having to target mineral deposits at increasingly
greater distances to the primary mineralisation.

In order to
see through the Earth’s cover and detect even subtle footprints of economic
mineral systems, researchers are using a variety of techniques, including mineral
geochemistry, to aid exploration efforts. Indicator minerals such as magnetite from
stream sediments, glacigenic sediments, or regolith cover can provide vectors
toward mineralised areas in environments where outcrops are scarce and access
is difficult.

Recent
years have seen an increased interest in the use of magnetite for provenance
studies and as a pathfinder mineral for exploration.

A recent
CSIRO study found that magnesium, aluminium, titanium, vanadium, chromium, manganese,
cobalt, nickel, zinc, gallium and tin concentrations display systematic
variations in magnetite from barren and mineralised rocks from different types
of mineral deposits. In addition, the occurrence, abundance and composition of
mineral inclusions in magnetite can also be a useful guide for exploration. For
example, sulphide inclusions in magnetite are a characteristic feature for
hydrothermal magnetite from sulphidic hydrothermal mineral deposits such as
skarn or porphyry systems.

Figure 1 3-D block diagram illustrating the
various settings in which indicator minerals such as magnetite can be employed
for provenance studies and mineral exploration.

Magnetite
is an important indicator mineral and commonly occurs in a variety of mineral
deposits and their host rocks. It crystallises over a wide range of geological
conditions and can incorporate a large number of minor- and trace-elements. Furthermore,
magnetite is more resistant to weathering and transport than many other minerals,
is easily identifiable, and can be easily magnetically separated due to its
magnetic properties. These features make magnetite an ideal petrogenetic
indicator. In combination with the development and improvement of analytical
techniques such as laser ablation inductively coupled plasma mass spectrometry
(LA-ICP-MS) and electron probe microanalysis (EPMA), that allow measurements
with increasingly lower detection limits, hydrothermal and igneous magnetite
can now be characterised in greater detail than previously possible. In-situ measurements on individual
samples offer valuable information that complement small–scale petrographic
observations, whereas bulk magnetic separates can be analysed rapidly with EPMA
and LA-ICP-MS to provide indepth data that can be used to reveal large-scale trends
in magnetite from stream sediments, regolith cover or drill core. “Such data
can provide key insights into the likely locations of ore deposits and the type
of mineralisation.”

Variations
in the concentrations of minor- and trace-elements in magnetite reflect the
formation conditions and the evolution of a specific geological setting and
represent a unique compositional signature. The composition of magnetite is
governed by a number of factors such as temperature,
fluid composition, oxygen and sulphur fugacity, silicate activity, host-rock
buffering, re-equilibration processes, and intrinsic crystallographic controls
such as ionic radius and charge balance. These factors translate to a unique magnetite
composition that can
help explorers discriminate mineralised from barren rocks in greenfields
exploration.

Figure 2 Median element concentrations in
parts per million in hydrothermal and igneous magnetite from a variety of
mineral deposits. Magnetite from the Inner Zone Batholith represents an example
from an unmineralised porphyritic host rock.

A
combination of multi-element statistics and element ratio plots can reliably
identify magnetite from different types of mineral deposits and discriminate
hydrothermal from igneous magnetite. Statistical data exploration is becoming
an increasingly invaluable tool to reveal trends and patterns in large
data sets. Explorers can use principle component or factor analysis and
discrimination measures to determine underlying trends and multi-element inter-relationships
that are often obscured in standard geochemical data processing and visualisation.

The composition and mineral inclusion
inventory of magnetite is a cost-effective and reliable tool that can help
explorers to target prospective areas in remote and deeply covered terranes.

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